Magnetic materials come in at least two forms, hard or soft. Hard magnetic materials are permanent magnets, which retain their magnetic properties after an energizing field is removed. An example of a hard magnetic material is a common refrigerator magnet. In contrast, soft magnet materials have a magnetic field which collapses after the energizing field is removed. Examples, of soft magnetic materials include electromagnets. Soft magnetic materials are widely used in electric circuits as parts of transformers, inductors, inverters, switch power supplies, and other applications. Soft magnetic materials are also used to make magnetic cores that provide high-energy storage, fast energy storage and efficient energy recovery. In these and other applications, magnetic cores may be used at a variety of different operational frequencies, typically ranging from 50 Hz to 20 kHz or more.
Most magnetic cores are made by winding a very thin magnetic metal strip or ribbon tightly around a substrate to form a multi-layered laminate. The wound metallic core is then subjected to a heating step, known as “annealing,” to optimize its performance through heat-induced ordering of the magnetic domains in the metal. After the annealing step, the substrate may be removed and the magnetic core may be treated with binding agents to hold the adjacent metal layers together so that the core will not unwind. As known to those of skill in the art, such binding agents may include epoxies, having either one or two parts, such as Hysol #4242 resin and #3401 hardener (Olean, N.Y.), or #2076 impregnation epoxy by Three Bond Co. Treatment with a binding agent also permits the core to be processed by cutting to form C or E cores, so named because the resulting cut cores resemble a C or an E, as known to those of skill in the art.
The metal strips or ribbon layers making up a magnetic core are very thin, typically from about 0.01 to 0.3 millimeters thick. For high frequency applications of greater than 400 Hz, the individual metal layers of a wound magnetic core must also be electrically insulated from one another for the core to function properly. Without such insulation, at high frequency the magnetic core has electrical properties similar to a large metal block, and will experience large power losses due to eddy currents.
To provide insulation between layers, the prior art generally teaches coating the metal ribbon with an insulating material prior to winding the ribbon to form the core. The insulating material is typically coated on both sides of the ribbon, and functions to insulate the metal layers in the wound laminate from adjacent metal layers. One widely used coating method is described in U.S. Pat. No. 2,796,364 to Suchoff, which discloses a method of forming a layer of magnesium oxide on a metal ribbon surface as an insulating layer. As described in Suchoff, magnesium methylate is dissolved in an organic solvent, and the solution is applied to the metal ribbon surface. The metal ribbon is then heated to high temperature to form a strongly adherent magnesium oxide insulating film over the surface of the metal ribbon. The metal ribbon may then be wound to form the magnetic core.
There are several known disadvantages to the magnesium methylate process. First, the magnesium methylate must be applied to the metal ribbon before it may be wound into a core. Uncoiling the metal ribbon, dipping the ribbon into a bath to form the coat, heating and curing the coat, and winding the ribbon to form the core make the process slow and expensive. The magnesium methylate process is therefore not suitable to provide insulation to magnetic cores in low cost, high volume applications. Second, it is very difficult to control the thickness of the resulting magnesium oxide insulating layer. This presents a problem for certain magnetic core applications, such as pulse cores, which have high performance specifications that are difficult to achieve unless the coated magnesium methylate layer is very thin. Forming thin magnesium methylate coatings requires special processing that is very slow and difficult to control. Use of the magnesium methylate process for these applications is extremely expensive, and the resulting cores are fragile. Furthermore, even for applications where a thicker insulating layer is acceptable, valuable magnetic core space is taken up when excessive nonconductive insulating material is present. This reduces the space factor of the laminated stack so that the percentage of the core occupied by magnetic material is lessened along with the efficiency of the core. Finally, because the magnesium methylate must be coated before the annealing step, it may also interfere with the ordering of magnetic domains during annealing by inducing stress buildup between the coating and the soft magnetic material.
The magnesium methylate process also cannot be used to form insulating layers for certain types of magnetic cores. High temperatures are required to properly cure the magnesium methylate on the metal ribbon. Typically, the magnesium methylate coating must be heated to temperatures of at least 843° C. (1550° F.) or more to form a magnesium oxide film which firmly adheres to the metal ribbon. However, some soft magnetic materials, such as amorphous metal alloys, may not be heated to temperatures greater than about 449° C. (840° F.) without destroying their desirable magnetic properties. When magnesium methylate is used as an insulating material for these types of metal alloys, it is heated to much lower temperatures, and the resulting magnesium oxide layer is only loosely bound to the metal ribbon. As a result, these types of cores may not be cut to form C or E cores, because the stressful cutting operation will cause the loosely bound insulating coatings to delaminate. Only uncut cores such as toroids can be formed from amorphous metal alloys coated with the magnesium methylate process. Moreover, the present inventors know of only one other process which may be used to form C or E magnetic cores of amorphous metal alloys. That process involves forming a thin discontinuous magnesium oxide coating on the ribbon prior to winding, and because the coating is not continuous, results in cores having high power dissipation at high frequency.
Thus, there is a need for improved methods of forming thin dielectric insulation on soft magnetic metal ribbons used to make magnetic cores. There is also a need for an insulation which permits processing of amorphous metal cores to form C and E cores that can be used at high frequencies.